An internal combustion engine has poppet valves including an intake valve for selectively opening and closing an intake port and an exhaust valve for selectively opening and closing an exhaust port. These valves are heated to a high temperature by heat produced in the combustion chambers. Therefore, if fuel or oil adheres to the valves, volatile elements of the fuel or oil vaporize and thus form a deposit. The deposit may accumulate on the heads of poppet valves, thus decreasing the amount of intake air flowing into the combustion chambers or increasing the flow resistance of the exhaust air flowing out from the combustion chamber. To avoid these problems, a poppet valve described in Patent Document 1 restrains adhesion of fuel or oil on a back of a head by employing a cover for covering the back. Also, a heat insulating space is provided between the cover and the poppet valve to decrease the amount of the heat transferred from the poppet valve to the cover, thus restraining a temperature rise in the cover. As a result, deposit accumulation on the cover is restrained.